Additive color digital image display devices are well known and are based upon a variety of technologies such as cathode ray tubes, liquid crystal modulators, and solid-state light emitters such as Organic Light Emitting Diodes (OLEDs). Devices such as solid-state lamps are also being produced. In a common additive color display device, a pixel includes red, green, and blue colored subpixels. These subpixels correspond to color primaries that define a color gamut. By additively combining the illumination from each of these three subpixels, i.e. with the integrative capabilities of the human visual system, a wide variety of colors can be achieved. In one technology, OLEDs can be used to produce color directly using organic materials that are doped to emit energy in desired portions of the electromagnetic spectrum, or alternatively, broadband emitting (apparently white) OLEDs can be attenuated with color filters to achieve red, green and blue.
It is possible to employ a white, or nearly white, subpixel along with the red, green, and blue subpixels to improve power efficiency or luminance stability over time. Other possibilities for improving power efficiency or luminance stability include the use of one or more additional non-white subpixels, such as yellow subpixels. However, images and other data destined for display on a color display device are typically stored or transmitted in three channels, that is, having three signals corresponding to a standard (e.g., sRGB) or specific (e.g., measured CRT phosphors) set of primaries. Therefore incoming image data will have to be converted for use on a display having four subpixels per pixel rather than the three subpixels used in a three channel display device.
In the field of CMYK printing, conversions known as undercolor removal or gray component replacement are made from RGB to CMYK, or more specifically from CMY to CMYK. At their most basic, these conversions subtract some fraction of the CMY values and add that amount to the K value. These methods are complicated by image structure limitations because they typically involve non-continuous tone systems, but because the white of a subtractive CMYK image is determined by the substrate on which it is printed, these methods remain relatively simple with respect to color processing. Attempting to apply analogous algorithms in continuous tone additive color systems would cause color errors if the additional primary is different in color from the display system white point.
In the field of sequential-field color projection systems, it is known to use a white primary in combination with red, green, and blue primaries. White is projected to augment the brightness provided by the red, green, and blue primaries, inherently reducing the color saturation of some or all of the colors being projected. A method proposed by Morgan et al. in U.S. Pat. No. 6,453,067 teaches an approach to calculating the intensity of the white primary dependent on the minimum of the red, green, and blue intensities, and subsequently calculating modified red, green, and blue intensities via scaling. However, the scaling cannot restore, for all colors, all of the color saturation lost in the addition of white. The lack of a subtraction step in this method ensures color errors in at least some colors. Additionally, Morgan's disclosure describes a problem that arises if the white primary is different in color from the desired white point of a display device, but does not adequately solve the problem. The method simply accepts an average effective white point, which effectively limits the choice of white primary color to a narrow range around the white point of the device.
A similar approach is described by Lee et al. (“TFT-LCD with RGBW Color System”, SID 03 Digest, pp. 1212-1215) to drive a color liquid crystal display having red, green, blue, and white pixels. Lee et al. calculate the white signal as the minimum of the red, green, and blue signals, then scale the red, green, and blue signals to correct some, but not all, color errors, with the goal of luminance enhancement paramount. The method of Lee et al. suffers from a similar color inaccuracy to that of Morgan.
In the field of ferroelectric liquid crystal displays, another method is presented by Tanioka in U.S. Pat. No. 5,929,843. Tanioka's method follows an algorithm analogous to the familiar CMYK approach, assigning the minimum of the R, G, and B signals to the W signal and subtracting the same from each of the R, G, and B signals. To avoid spatial artifacts, the method teaches a variable scale factor applied to the minimum signal which results in smoother colors at low luminance levels. Because of its similarity to the CMYK algorithm, it suffers from the same problem cited above, namely that a white pixel having a color different from that of the display white point will cause color errors.
Primerano et al., in U.S. Pat. No. 6,885,380, and Murdoch et al., in commonly-assigned U.S. Pat. No. 6,897,876, the disclosures of both of which are incorporated by reference herein, describe methods for transforming three color-input signals (R,G,B) into four color-output signals (R,G,B,W) which do not cause color errors when the white pixel has a color different from that of the display white point. Although useful, these methods assume that the color of the emitters and in particular the color of the W emitter (white, in these cases) is constant.
As described by Lee et al. in US 2006/0262053, the color of a white-emitting OLED can change with the controlling voltage. In other words, the color of a white-emitting OLED can vary with the intensity of emission. This problem can affect white subpixels in OLED or EL displays. It can also affect OLED or EL lamps, which can be considered to include a single, very large white subpixel. While a number of other methods have addressed the problem of transforming three color-input signals to four color-output signals, e.g., Morgan et al. in U.S. Pat. No. 6,453,067, Choi et al. in US 2004/0222999, Inoue et al. in US 2005/0285828, van Mourik et al. in WO 2006/077554, Chang et al. in US 2006/0187155, and Baek in US 2006/0256054, these methods cannot adjust for a white emitter with variable color. While Lee's method can adjust for a white emitter with variable color, it requires a set of six coefficients to apply a correction after the conversion from three color signals to four color signals. This method is computationally and memory intensive, and would be slow and difficult to implement in a large display. Gathering data for the method requires manual adjustments that can be time-consuming and labor-intensive. It requires gathering spectral data, which is more complex and time-consuming than colorimetric measurements. Further, it does not mathematically provide a colorimetric match between a desired RGB color and the RGBW equivalent.
Co-pending commonly-assigned U.S. Patent Application Publication No. 2008/0252797, filed Apr. 13, 2007, entitled “Method for input-signal transformation for RGBW displays” by Hamer et al., the disclosures of which are incorporated by reference herein, describes a method for transforming RGB to RGBW, where the W has color that varies with drive level.
US Patent Application Publication No. 2009/0189530 by Ashdown et al. describes feedback control of RGB LEDs by superimposing AM modulation on the PWM drive signal. However, the AM modulation does not provide control of chromaticity or luminance. It serves only to differentiate the R, G and B channels when sensed by a single photosensor.
US Patent Application Publication No. 2008/0185971 by Kinoshita describes adjusting current density and duty cycle of an EL emitter independently to vary chromaticity while keeping luminance constant. However, this scheme is limited to only chromaticities the EL emitter can produce natively. This is not sufficient for full-color displays, in which the desired chromaticity may not lie on the chromaticity locus of the EL emitter.
There is a need, therefore, for an improved method for compensating for chromaticity shift of an EL emitter in a single- or multi-color EL device or display.